1. Field of the Invention
This invention relates to threaded connections for elongated pieces of pipe normally subjected to high fluid pressures, such as oil and gas well pipe, especially well casing, and more particularly to a well casing connection or coupling having improved strength and leak tightness and in which the sealing surfaces are brought into pressure contact without relative rotating movement therebetween.
2. Background of the Prior Art
In oil and gas well drilling operations it is common practice to provide metal casing in the form of a number of steel pipe sections connected by threaded connections which are ordinarily screwed together at the rig floor during what is referred to in the art as the "stabbing" and "make-up"procedure. The casing serves to define the bore hole, and thereby prevents the surrounding earth from entering the hole and also prevents the well products from escaping into the surrounding earth strata. Additionally, the casing serves to contain the drilling fluids and other compounds often used in the bore hole to cool or lubricate the drill and thereby speed up the drilling operation, and it also serves to facilitate the removal of the drilling fluids and compounds, and also of the oil or gas when the drilling operation is completed.
The casing includes a plurality of interconnected pipes or "joints," the connections generally being effected by threaded couplings that engage with corresponding threads on the ends of the adjacent casing sections. The resulting assembled casing is often called a string, and it extends from the floor of the drilling platform or rig down to the bottom of the well hole. When the casing is made up at the rig floor, one joint or section of pipe is already partially in the bore hole with an internally threaded female end of a connector extending slightly above the rig floor. The next joint or section of pipe, which has an externally threaded male end, is elevated and held vertically in a derrick with the lowermost end above the female end of the adjacent pipe. The uppermost joint of pipe is then lowered with the male end directed into the female end so that the threads can be engaged, or, as it is often expressed in the art, the uppermost joint is "stabbed" into the lowermost joint.
Because the "strings" of pipe are of substantial length, the connections must be sufficiently strong to withstand the bending loads imposed thereon, while at the same time providing a fluid-tight connection. The connections must also be capable of withstanding high tensile and compressive loads, the tensile loads being reflected principally in the uppermost connections of the casing, and caused by the weight of the lower portions of the casing, which act upon the connections above them and spaced along the vertical extent of the casing. Additionally, if an obstruction is encountered as the drilling operation proceeds, the connections are often subjected to compressive loads that are imposed on the casing from above in order to assist the drill and casing to penetrate and pass through the obstruction.
Over the course of the years a number of different types of thread and connector arrangements have been developed for oil well casing connections. For example, an arrangement utilizing cooperating tapered, threaded surfaces is disclosed in U.S. Pat. No. 3,854,760, which was granted on Dec. 17, 1974, to Jean Duret, in which a thread having an asymmetrical cross section and rounded edges is disclosed, and wherein the respective ends of the adjacent pipes are similarly configured for close engagement along cooperating and interengaging inclined surfaces formed on the respective ends thereof.
In U.S. Pat. No. 2,783,809, granted on Mar. 5, 1957, to M. W. Haines, a pipe union is shown and described including a tubular coupling connector and pipe ends having interengaging, tapered sealing surfaces. The tubular connector has a coarse, Acme-type thread formed on the interior thereof, with opposite faces of the thread cut to different leads. The pipe ends have external threads of the same hand and with leads that respectively correspond with those of the opposite faces of the internal threads on the inside of the connector with which they are adapted to be engaged. In operation, when turned, the connector serves to exert compression forces at the previously coupled sealing surfaces of the pipe ends.
Although a number of thread and connector configurations for oil field pipe have been developed over the course of the years, the use of available pipe-end-to-pipe-end sealing arrangements has not always provided a strong and fluid-tight seal under all the various conditions to which the connections are subjected in use. Additionally, in order to provide a fluid-tight joint, it is necessary to apply a high torque to one section of the pipe while holding the other section stationary in order to cause firm engagement between the respective sealing surfaces of the two joints. However, the rotation of one sealing surface relative to and against the other sealing surface while high compression forces are being applied to the surfaces frequently causes those surfaces to gall, or to wear away by the rubbing together of the sealing surfaces, thereby reducing the effectiveness of the seal therebetween.